1. Field of the Invention
The present invention relates generally to display systems and, more particularly, to systems for processing video signals.
2. Description of the Related Art
The viewability of an image on a display is generally determined by the brightness and contrast of the display and video signal corresponding to the image. The luminance of each display pixel corresponds to the amplitude of the video signal for the pixel. High amplitudes typically correspond to very bright pixels, while low amplitudes generally correspond to dark pixels. The range between the minimum and maximum amplitudes and the corresponding degrees of luminance may be subdivided into an almost infinite number of luminance levels, reflecting subtleties of shading and color represented by the video signal.
The display's brightness and contrast adjustments, on the other hand, arc essentially static. Conventionally, brightness corresponds to a DC signal added to the video signal so that the overall signal level increases. As a result, the overall display becomes brighter. For CRT displays, the DC component is added to the video signal. For liquid crystal displays (LCDs), a backlight system responds to the brightness control.
Contrast, on the other hand, relates to the amplification of the video signal. Thus, as contrast increases, bright pixels become very bright, while relatively dark pixels become only slightly brighter.
The brightness and contrast controls allow the user to manually adjust the viewability of the data. Conventional display systems may also automatically compensate for brightness variations due to transmission loss. Conventional video signals typically include a sync signal which display circuitry may use to determine transmission loss. Typically, the video signal includes a sync signal having a specified magnitude, such as 2.86 volts. When the display receives a video transmission, it compares the measured sync signal amplitude to the target amplitude to determine the transmission loss. For example, if the target sync signal has an amplitude of 2.86 volts and the received sync signal was only 1.43 volts, the display logic assumes that the entire signal is also attenuated due to transmission loss and should amplified by a factor of two.
Conventional video displays, such as cathode ray tube (CRT) displays, also typically have a wide dynamic range (i.e., a number of different and distinguishable colors and shades) for displaying each pixel with the appropriate degree of brightness according to the video signal and the brightness and contrast criteria. Small increases in amplitude cause small increases in brightness, regardless of whether the increase is due to a change the video signal or the brightness or contrast control. Consequently, subtle differences in the video signals induce subtle differences in the picture rendered by the display.
In some applications, however, subtle differences are not apparent to the user. For example, in some radar-based imaging applications, the dynamic range or peak-to-peak variation of the video signal information is relatively small. A CRT display shows variations in the video signal as slightly different shades. Where the variations are very small, the differences between different shades in the image may be so slight as to be nearly imperceptible.
This problem is compounded for various modern displays which do not provide the broad dynamic range of CRT displays. Limitations in a display's dynamic range may restrict, or even negate, the display of subtleties in the image. For example, while the dynamic range of various LCDs varies according to type and manufacturer, LCDs generally have a limited dynamic range, particularly in comparison to CRT displays. A typical LCD exhibits a dynamic range limited to, for example, 64 or even 16 shades of gray.
For displays with limited dynamic range, effectively displaying and viewing minor variations in the data or information content is difficult, if not impossible. With limited dynamic range, slight variations in the video signal are commonly lumped into the same shade. As a result, variations in the video signal may not affect the rendered image at all, potentially obscuring vital information. Thus, it would be advantageous to provide a system for utilizing the available dynamic range of a display to enhance the presentation of data.